Solid electrolyte sheet, method for producing same, and all-solid-state secondary battery

Provided are: a solid electrolyte sheet containing a sodium ion-conducting crystal, which is capable of obtaining excellent battery characteristics even when the sheet is small in thickness; and an all-solid-state battery using the solid electrolyte sheet. The solid electrolyte sheet is characterized by containing at least one sodium ion conductive crystal selected from the group consisting of beta ''-alumina and sodium super ion conductor crystals, and the solid electrolyte sheet has a thickness of 500 [mu] m or less and a flatness of 200 [mu] m or less. 本发明提供一种即使厚度小也能够得到优异的电池特性的含有钠离子传导性结晶的固体电解质片和使用其的全固体电池。该固体电解质片的特征在于：含有选自β"-氧化铝和钠超离子导体结晶中的至少1种钠离子传导性结晶，该固体电解质片的厚度为500μm以下且平面度为200μm以下。